1. Field of the Invention
The present invention relates to a method for developing an electrostatic latent image, and more particularly to an image developing method in which an electrostatic latent image formed by electrophotography, electrostatic recording or the like method is developed using a two component developer including a carrier and a negatively charged toner to form a visual image. In addition, the present invention also relates to a developing roller and a developing device therefor.
2. Discussion of the Background
Various electrophotographic image forming methods have been disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publication No. 42-23910 (i.e., U.S. Pat. No. 3,666,363) etc.
In these image forming methods, visual images are typically formed as follows:
(1) an electrostatic latent image is formed on a photoreceptor, for example, by charging the photoreceptor and then irradiating the photoreceptor with imagewise light;
(2) the electrostatic latent image is developed with a toner to form a toner image on the photoreceptor;
(3) the toner image is transferred on a receiving material such as receiving paper; and
(4) the toner image on the receiving paper is fixed thereon upon application of heat, pressure and/or a vaporized solvent thereto.
With respect to the method for developing an electrostatic latent image using a toner, various developing methods are known. For example, the following methods have been disclosed:
(1) developing methods using a magnetic brush (e.g., U.S. Pat. No. 2,874,063)
(2) cascade developing methods (e.g., U.S. Pat. No. 2,618,552);
(3) powder cloud developing methods (e.g., U.S. Pat. No. 2,221,776);
(4) developing methods using a fur brush; and
(5) developing methods using a liquid toner.
Among these developing methods, magnetic brush developing methods and cascade developing methods, which use a two component developer including a toner and a particulate carrier, and liquid developing methods have been widely used.
In addition, Japanese Laid-Open Patent Publication No. 52-94140 (i.e., West German Patent No. 2,704,361) discloses a developing method utilizing dielectric polarization of a magnetic toner having a high electric resistance.
Further, developing methods utilizing frictional charging of a magnetic toner having a high resistance have been disclosed. In the developing methods, an electrostatic latent image is developed while being contacted with particles of the magnetic toner, which are typically charged by being rubbed with other toner particles, a developing sleeve and the like material, resulting in formation of a toner image.
As for the developing method using a two component developer, various methods are known. However, a typical developing method using a two component developer is as follows:
(1) a toner and a magnetic carrier are mixed and agitated with an agitator to charge the toner and carrier in a hopper of a developing unit; and
(2) the mixture of the toner and carrier in the hopper is fed little by little to the developing portion of a photoreceptor by a magnetic roller such that fresh toner particles are supplied to the developing portion;
The hopper may include only a toner. In this case, the toner in the hopper is fed little by little to a magnetic roller on which a mixture (i.e., a developer) of the toner and a carrier is held. The toner and the carrier are mixed and agitated on the magnetic roller, and thereby the toner particles in the developer become to have a charge.
In these developing methods, the toner may be a magnetic toner or a non-magnetic toner. Namely, developing methods using a two component developer include developing methods using a magnetic two component developer and developing methods using a non-magnetic two component developer.
In addition, the toner may have a negative charge or positive charge. If a toner to be charged negatively is used, the toner particles in a thin developer layer formed on a developing roller have to be fully charged negatively. If the toner particles on the developing roller are not fully charged negatively, i.e., if toner particles having a positive charge are present in the developer, a background development problem occurs in which the background area of a toner image is undesirably developed with the positively charged toner particles.
In order to avoid such a background development problem, the following subjects should be investigated.
(1) What is the best material for the surface of the developing roller and/or carrier to effectively impart a frictional charge to toner particles?
(2) What is the best condition of the surface of the developing roller and/or carrier particles to effectively and uniformly impart a frictional charge to toner particles?
However, such subjects have not yet been investigated.
Therefore, the surface of a developing roller is designed after repeating trial and error. Accordingly, toner particles cannot be stably charged uniformly, and toner particles having an opposite charge are produced, resulting in occurrence of the background development problem. In addition, when the background development problem occurs, toner consumption increases, resulting in increase of running costs of image forming apparatus. Therefore, it inflicts a loss on users.
Because of these reasons, a need exists for a developing method by which images having good image qualities can be stably produced without causing the background development problem.
Accordingly, an object of the present invention is to provide a developing method by which images having good image qualities can be stably produced without causing the background development problem.
Specifically, an object of the present invention is to provide a developing roller which is configured to bear a two component developer while charging toner particles in the developer so that the toner particles have a uniform charge.
Another object of the present invention is to provide a developing roller and a developing device which develop an electrostatic latent image and which can produce good toner images without causing the background development problem.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a method for developing an electrostatic latent image including the step of developing an electrostatic latent image with a two component developer including a particulate carrier and a negatively charged toner and held on the surface of a developing roller to form a toner image, wherein the developing roller surface includes a material including one or more carbon atoms and one or more nitrogen atoms, and wherein a ratio of the number of carbon atoms to the number of nitrogen atoms is not greater than 100/1.
The material in the surface of the developing roller may be a resin, or a combination of a binder resin and a low molecular weight compound including one or more carbon atoms and one or more nitrogen atoms, wherein the ratio of the carbon atoms to the nitrogen atoms is not greater than 100/1 by number.
In another aspect of the present invention, a two component developing method is provided which includes the step of developing an electrostatic latent image with a two component developer held on a developing roller to form a toner image, wherein the two component developer includes a particulate carrier and a negatively charged toner, wherein the particulate carrier has a surface layer including a material including one or more carbon atoms and one or more nitrogen atoms, and wherein the ratio of the number of carbon atoms to the number of nitrogen atoms is not greater than 100/1. In this method, the developing roller may be the developing roller of the present invention mentioned below.
In yet another aspect of the present invention, a developing roller is provided which develops an electrostatic latent image by a two component developer including a carrier and a negatively charged toner and held on the surface of the developing roller, wherein the surface of the developing roller includes a material including one or more carbon atoms and one or more nitrogen atoms, wherein the ratio of the number of carbon atoms to the number of nitrogen atoms is not greater than 100/1.
In a further aspect of the present invention, a developing device is provided which develops an electrostatic latent image and which includes a two component developer including a particulate carrier and a negatively charged toner; a developing roller which bears the two component developer on the surface thereof to develop the electrostatic latent image to form a toner image, wherein the surface of the developing roller includes a material including one or more carbon atoms and one or more nitrogen atoms, wherein the ratio of the number of carbon atoms to the number of nitrogen atoms is not greater than 100/1.
In this developing device, the surface of the carrier may have the surface layer mentioned above.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawing.